A general optical receiver in optical communication is generally formed by a light-receiving element and a transimpedance amplifier (to be also referred to as a “TIA” hereinafter) that amplifies a photocurrent generated by the light-receiving element.
Examples of the light-receiving element used in the optical receiver are a photodiode (to be also referred to as a “PD” hereinafter) and an avalanche photodiode (to be also referred to as an “APD” hereinafter).
The PD has a function of converting incident light to a current. The upper limit of the photoelectric conversion efficiency of the PD is 100% as quantum efficiency. As the PD, a uni-traveling carrier photodiode (UTC-PD) and the like are known in addition to a general element made of a III-V compound semiconductor such as InGaAs (see, for example, non-patent literature 1).
On the other hand, the APD is a light-receiving element having a function of making photoelectrons generated in the element hit a lattice by accelerating them under a high electric field and thus ionizing the photoelectrons, thereby amplifying the carriers. The APD can output a plurality of carriers in correspondence with one photon, and thus obtain a sensitivity higher than 100% as the quantum conversion efficiency. For this reason, the APD is widely used for a high-sensitivity optical receiver (see, for example, non-patent literature 2).
In recent years, for the purpose of downsizing an optical receiver and reducing the cost of the optical receiver, research and development of monolithic integration of integrating an optical waveguide including an optical multiplexer and an optical demultiplexer, a light-receiving element, a TIA, and the like into a single IC chip are attracting attention. Particularly, “silicon photonics” of sharing a silicon (Si)-based IC and a manufacturing process and manufacturing an optical active element such as a light-receiving element has been extensively researched and developed (see, for example, non-patent literature 3).
By applying a silicon photonics technique to an optical receiver, the integration and formation of a light-receiving element and a CMOS (Complementary Metal Oxide Semiconductor) circuit on silicon (Si) or SOI (Silicon On Insulator) together become possible. Thus, it is possible to reduce the cost in terms of the mass productivity of the optical receiver, the stability of the manufacturing process, packaging, and inspection.
In recent research and development of the optical receiver by silicon photonics, a method of performing, on a silicon substrate, crystal growth of germanium (Ge) having sensitivity in a 1.3-μm band and having a relatively small difference in lattice constant with respect to silicon, a method of growing, on an InP substrate, InGaAs functioning as a light absorption layer and then transferring InGaAs onto an Si substrate by, for example, bonding, or the like is used. For the purpose of improving the sensitivity of the light-receiving element, an APD including a multiplication layer made of silicon (Si) has also been researched and developed.
As an APD by silicon photonics, a “vertical incident type” APD having a structure for stacking a light absorption layer on an Si or SOI substrate and performing voltage application and light injection in a direction parallel to the stacking direction, or a “normal incident type” APD is known.
In addition to the above APDs, a waveguide type APD by silicon photonics is known. The waveguide type APD has the feature that an optical waveguide and a light-receiving unit can be integrated and it is unnecessary to use a spatial optical system at the time of implementation. As a waveguide type APD, for example, non-patent literature 4 discloses an APD whose degree of integration of devices is improved by accelerating light absorption by forming a waveguide in an Si substrate and injecting a fault into the waveguide, and composing, by only Si, a material for applying an electric field in the waveguide and multiplying it.
Non-patent literature 5 discloses a waveguide type APD in which a contact layer is provided in each waveguide having functions as a Ge light absorption layer and an Si multiplication layer and evanescent coupling is used for optical coupling between the Ge light absorption layer and the waveguide.